SOURCE

let a = {};
Object.defineProperty(a,'b',{
  get:function(){
    console.log('get runs')
    return this.value
  },
  set:function(newval){
    console.log('set runs');
    this.value = newval
  }
})

a.b = [1,2,3]; //set runs
a.b.push(5);  // get runs